Continuous casting in general is a process by which the uninterrupted flow of molten metal enters one end of the casting apparatus, is solidified, and is withdrawn at the other end having the desired configuration. Continuous casting of steel for high quality steel products, such as pipes, tubes and the like, of necessity must be carefully controlled. For example, the casting temperature of the molten metal has to be maintained within a certain range during the cast. To assure the internal and surface quality of the finished product, defects, such as porosity, cracks, segregation and inclusions of the casting, should be minimized or be such that they are not harmful. This requires controlled feeding of liquid metal, casting speed and cooling rate.
Continuous cast steel usually is hot deformed in further processes to produce a homogeneous end product with acceptable quality and soundness. The deformation from the cast size to finished size usually would be at least 4:1 and could be as high as 15:1 for some products. To withstand successfully such further deformation while desired quality of the end product is achieved, the cast material must be of high quality with minimum defects as aforesaid.
For reasons of economy it is desirable to maximize the amount of liquid steel cast as a batch or heat. Heats of 200 tons of liquid steel are common. Heats of 300 tons or more are in operation or proposed. To cast such large amounts of liquid steel within a certain time and at an acceptable speed, multiple strands per machine are required; but it has been found that the maximum should not exceed six strands from the operational and engineering standpoint. For example, to cast 200 tons of liquid steel the casting time should not be more than 90 minutes. For solid billets casting speeds of over 100 inches per minute are reported but most machines have casting speeds between 30 and 80 inches per minute.
In modern casting machines for steel a distributor or tundish feeds molten metal into the molds of the individual strands. Metering is by stopper rod or sliding gate made of high temperature refractory. A water cooled oscillating mold initiates the solidification process. As soon as a crust of sufficient thickness and strength is formed by the mold, water spray cooling is applied to complete the solidification process.
The cross section of the cast steel is determined by the intended subsequent deformation to finished product and the casting mold and an adequate opening in the tundish to feed steel into the mold are, accordingly, appropriately sized. The tundish opening should be sufficient to prevent clogging. Furthermore, a large mold size favors flotation of liquid steel impurities which easily could be trapped in the solidifying steel. It is generally agreed that the minimum thickness of cast steel should not be less than 4 inches.
Since continuous casting has proven to be an energy saving process for steel production with higher overall productivity for capital investment and labor than other steel manufacturing processes, it has found increasing acceptance. Approximately 21% of the world steel production in 1979 was continuously cast, but the percentage was less for the steel pipes.
For relatively high production of seamless steel pipes from solid round billets, conventionally such solid billets are pierced to form hollow round billets (or hollow rounds) which then are further processed by various operations to form finished pipes. The solid round billets themselves are produced either by the traditional ingot process with the ingots being rolled to billet size or by casting billets on a continuous casting machine. Billets can be cast to size or cast at a relatively larger section and then rolled to desired billet size. One problem using such technique, though, is that continuously cast billets usually have internal defects generally concentrated towards the axial center thereof. Such defects are a cause for inside quality problems of the finished pipe produced therefrom.
To avoid using the above mentioned piercing process and its attendant disadvantages in forming hollow round from solid round, attempts have been made to cast directly hollow round billets. The aforesaid defects, then, would be confined internally of the billet wall, i.e. not at the external or internal wall surfaces of the hollow billet. Therefore, such internally confined defects would not detrimentally affect the inside or outside surface quality of the finished pipe.
In one prior casting machine hollow rounds were cast directly in a U-shape mold with withdrawal in an upward direction. Such machine was based on the principle of "Communicating Vessels" and no internal plug was required. However, such machine has not been widely commercially accepted.
Hollow round steel billets also have been cast by a centrifugal process. This process has found particular application for stainless steel and special alloy steel pipes with limited production.
On an experimental and low production basis hollow round has been cast using an internal water cooled plug for primary inside cooling and water spray for secondary inside cooling. The plug normally oscillates with the outside mold. However, casting hollow round with such internal plug and spray cooling has encountered many problems. For example, internal explosions have occurred after an internal breakout when liquid steel has come in contact with water or steam. Venting and detecting devices, therefore, are necessary to avoid explosive conditions, but such devices increase the complexity of the equipment and do not necessarily assure that explosions will be avoided.
Another problem encountered by such latter type of direct casting machines has been the contracting of the inner annulus of the solidifying steel as it cools and the freezing thereof to the plug. The friction forces between plug and annulus, then, cause cracking and rehealing to the inside surface thereof, which is detrimental to the inside surface quality of the finished pipe. Tapered and corrugated plug surfaces in addition to the oscillation as well as an expandable/contractable plug have been suggested and tried to try to overcome these problems but have not been totally successful.
The as-cast macro-structure of the material formed by the prior techniques for direct cating of hollow rounds has not been particularly suitable for further processing in an elongator. Processing the hollow rounds in an elongator is the first step in high production mills to reduce the wall thickness. Two or three contoured rolls with inclined axes rotate the billet and advance it over an internal mandrel. The billet rotates around its axis with a surface speed between 800 and 1200 feet per minute imposing tangential stresses on the inside and outside surface by the centrifugal forces. Since the tangential stresses on the inside surface far exceed the stresses on the outside surface, the relative low strength of the macrostructured material often will rupture inside reducing the quality of the finished product. Hollow round billets cast with a plug and inside spray cooling have been processed in presses or in a pilgrim mill which forges the wall axially. After every forging process the billet is turned 90.degree.. These processes, though, are low production operations and are economical only in certain cases.
Further, it has been found that the wall thickness of the hollow round billets formed by prior direct casting techniques varies over the whole length. Such variations could detrimentally affect the wall tolerance of the finished pipe. Using additional plugs in the secondary cooling zone centered by a magnetic field has been suggested for improvement of wall thickness uniformity.
It will be appreciated that it would be desirable to eliminate the aforesaid problems encountered in making hollow round billets, tubes and the like, especially by direct continuous casting. Both external and internal surface defects should be avoided and good ductility should be maintained to permit facile, high speed elongation while quality of the finished product is held. It also is desirable to maximize production speed, to minimize machine space requirements and to minimize capital and labor costs.